Working with penises, semen and testicles is no laughing matter but a sense of humour is essential, says a bull whisperer.

Interposing yourself between an amorous bull and the object of its lust is a dangerous occupation, but for semen collector Robyn How, of the Tararua Breeding Centre in Woodville, it is a fascinating way of life.

Born and raised in Australia, How became passionate about cattle after helping a friend with show animals. While doing an artificial insemination course, she found she had a natural ability to read bulls.

She bought a 6ha lifestyle block in Woodville in 1997 and started the breeding centre the next year with Auckland-based business partner and embryo transfer veterinarian Eddie Dixon.

The centre collects semen from all types of bulls, which are trucked in from all over New Zealand. Some of the bulls are already proven when they arrive while others are there to be proven.

How says she shows them respect and although she has never been injured, she doesn't take any bull for granted.

''Having full respect, understanding and appreciation for the bulls is crucial. Semen collection is not a job anyone can do.

''We know just by reading an animal, how far we can push things. There is no way any of us could outrun a bull.''

Giving the bulls plenty of space is key to semen extraction, with each bull having their own paddock complete with a mound of dirt and scratching post.

''A happy bull produces happy semen. We want them to release any aggravation and testosterone on their mounds of dirt and posts before working with them.''

How says some bad-tempered bulls have been badly handled but with time and no pressure they can be turned around.

''The trick is to be patient and give them time and space to settle.

''I have collected semen from some pretty mean bulls,'' she says.

Ranging in age from seven months to 14 years, bulls can stay for as little as six weeks with some living at the centre permanently. The oldest bull was 15.

''We held grave concerns for the quality of his semen and his libido. The client asked us to give him a go and we weren't very confident, but he shocked us all and turned out to be the Hugh Hefner of the bull world.''

''We are invested in the bulls' wellbeing and take pride in our work. If we can send a bull home with a better temperament and in better condition than he arrived, then it is a win.''

Before arriving at the centre, animals have to be tested on-farm for tuberculosis and blood diseases. After three weeks on the centre's quarantine block, they are tested again for diseases and if clear, sent to the collection centre for semen production.

The centre processes about 4000 straws a day. Annually, more than 300,000 samples are processed.

The centre is full of bulls from May to December. All semen collected belongs to the bulls' owners. Some is returned to them and the rest stored for future dispatch or export. Around 65 per cent of the semen is exported.

Australia is the biggest recipient, followed by North America. South America is also a big importer.

How also runs Embryco in partnership with embryo transfer veterinarian Martin Hamer. It produces embryos for both local use and export. Seven days after cows are artificially inseminated, fertilised embryos are flushed out and frozen. They are then either exported or replanted in other animals around the country.

Another service provided is onfarm testing. Although How still does this when time permits, she has trained Guy Haynes from Totally Vets in Fielding to carry out this work. Haynes, a fully trained morphologist travels all over the country collecting bulls, testing their fertility and capability of impregnating cows and can carry out all semen assessments while on the road.

How says the best part of her job is that every day is different.

The bulls all have individual traits and have to be handled accordingly.

''At the end of the day, not everyone wants to work with penises, semen and testicles and while we can laugh about it, it is a serious business.